In oil and gas well drilling operations, occasionally objects become stuck in the well. For example, the object, often referred to as a fish once stuck, might have been a tool lowered into the well on a wire line that became stuck on a ledge or a collapsed section of the well, preventing its retrieval. When this occurs, the operator releases the line from the object by parting it at a weak point. Then the operator runs back in the well with a working string that may be wire line, coiled tubing, threaded tubing or drill pipe to retrieve the object. Often, a set of jars will be located in the working string to provide impacts to the object to help retrieve the object.
Generally there are two types of jars in use, hydraulic release and mechanical release. A hydraulic release jar has an orifice within it and is filled with a liquid. It is operated by pulling tension on the work string and waiting for sufficient fluid to bypass internally to allow the jar to reach internal release position. The jar then rapidly opens several inches, and energy stored in the accelerator and/or the tubing string is imparted to the engaged object. The operator then slacks off tension in the work string to repeat the cycle. The operator can vary the release tension without retrieving and adjusting the tool. However, hydraulic release jars are relatively expensive and not very dependable. They have a tendency to become contaminated by wellbore environments due to the high internal pressure differentials inherent to their operation.
Mechanical release jars, while being more dependable, must be adjusted on the surface to the anticipated release tension prior to being run in the hole. If these jars are set to a release tension which cannot be attained upon downhole engagement, or if the tension proves to be too low to be effective, the work string must be disengaged, pulled out of the hole, and readjusted.
Both types require a recocking by movement of the work string. This involves lowering work string tension at the tool to zero, then applying enough weight to overcome any inherent resistance to recocking in the tool. Other than observation of a weight indicator, there is no surface indication that recocking has occurred. If hydraulic release jars are being used, which are time delay devices, much time can be consumed waiting for jars to fire which have not been recocked. Further, operators tend to apply more weight than required during recocking to insure recocking occurs. There are two hazards in this practice. Applying weight at the fishing tool may cause the fishing tool to become disengaged, especially with a ratchet type mechanism. Also, downward firing jars may be fired inadvertently, applying unwanted or destructive down shock loads to the fishing tools or fish.
A major disadvantage of recocking the types of tools described above is the requirement for moving the work string up and down for each impact. Hundreds of jarring cycles may be needed before a fish release is obtained. If surface pressure is present, packoff devices must be stripped through each time the tool is cycled. If coiled tubing is used as the work string, correlation between depth and weight is easily lost. Wrapping and unwrapping of tubing on the reel, and variations in reel tension and friction in pressure control devices affect weight indicator readings and create uncertainty. The repeated cycles of wrapping and unwrapping of the coiled tubing cause fatigue and wear on the coiled tubing.
As mentioned above, the work string may be wire line, coiled tubing, threaded tubing, or drill pipe. In wells that are highly deviated, wire line will not function. Also, if circulation or high tensile loads are required, wire line is unacceptable. Coiled tubing has an advantage over threaded tubing and pipe because it is faster to rig-up and trip. If well pressure exists, surface pressure control is much less complex and more dependable with coiled tubing. The internal passage of coiled tubing is never exposed to the atmosphere because no making and breaking of connections is required. The operator can pump through coiled tubing at any time during the operation, even during tripping. The disadvantage of coiled tubing, as mentioned above, is the bending and straightening that occurs while the jar is being recocked. This bending and straightening induces fatigue, which accumulates locally until the tubing fails by breaking. Larger OD coiled tubing may fail with as few as thirty cycles. Common size coiled tubing, 11/4 to 11/2 inch, are limited to less than 200 cycles before failure. Even if the fish is retrieved prior to catastrophic failure, accumulated localized fatigue remains in the affected section of the work string. When coiled tubing fails during a workover operation, many problems, some of which may be dangerous, result. In any case, considerable time and expense are incurred in removing parted coiled tubing from a well. Because of the fatigue problem, if high tensile loads are required during a fishing operation, operators generally will not use larger OD coiled tubing and use threaded tubing, even though more time consuming.
One type of jar shown in prior patents does not require cycling of a work string to recock the jar. This tool is driven by hydraulic fluid pressure pumped down from the surface. In this type of jar, the liquid pumped down the string will cause a piston to move, compressing a main spring. When the spring is fully compressed, the piston is released with the main spring delivering an impact. This type avoids having to move a string of coiled tubing back and forth for each impact. However, it relies on the force of the main spring to deliver the impact, which may not be adequate in some cases.